Olefin polymers such as polypropylene, high-density polyethylene, linear low-density polyethylene (LLDPE) and the like are excellent in not only transparency but also mechanical strength such as rigidity and impact strength, and have been conventionally molded into films by means of inflation molding, injection molding, extrusion molding, etc.
Such olefin polymers as mentioned above generally are low in the melt tension (MT), so that it is difficult to mold them into large capacity containers (e.g., bottles) for example by blow molding or into liners of electrical appliances for example by a vacuum molding. By those restrictions in the molding processes, the resulting molded products are also restricted. That is, the use applications of the olefin polymers are restricted in spite that they have various excellent properties.
Further, as for polypropylene, there are such problems that a phenomenon of drawdown occurs and molding conditions are restricted when polypropylene is molded into a film by an inflation molding, because of its low melt tension. For coping with those problems, a method of adding a high-pressure low-density polyethylene or the like to polypropylene is carried out in the conventional inflation molding process to increase the melt tension thereby to stabilize bubbles. However, this method sometimes induces decrease of the film strength and decrease of a film transparency.
Accordingly, if olefin polymers (e.g., polypropylene) having a high melt tension are developed, it becomes possible to form large capacity containers such as bottles by a blow molding and to form liners of electrical appliances by a vacuum molding from those polymers, and hence the use applications of the olefin polymers can be much more extended.
Further, when the olefin polymers having a high melt tension are molded into films by means of an inflation molding, the bubbles can be stabilized and the molding speed can be made higher.
For these reasons, an advent of olefin polymers such as polypropylene, high-density polyethylene and linear low-density polyethylene having high melt tension has been eagerly desired.
The present inventors have studied the olefin polymers of high melt tension to comply with the above-mentioned requirements, and as a result, they have found that an olefin polymer of high melt tension can be obtained by polymerizing olefin in the presence of a catalyst for olefin polymerization comprising a novel prepolymerized catalyst which is obtained by copolymerizing an .alpha.-olefin and a polyene compound to a catalyst which comprises a transition metal compound catalyst component and an organometallic compound catalyst component, and accomplished the present invention.